This invention relates to a thin film capacitor which is useful as a thin film capacitance element in a semiconductor integrated circuit or in the peripheral circuit of semiconductor integrated circuit.
There has been a rapid progress in the integration of LSI, in particular DRAM (Dynamic Random Access read write Memory) whose integration has been increased by four times as compared with that of three years before. This trend of increasing the integration is expected to be further advanced in future in the same rapidness as in seen in the past few years. However, since it is required in the case DRAM to retain a prescribed capacitance in view of assuring the sensitivity of a sense circuit as well as in view of preventing a soft error by a radiation, the effective area of a capacitor within a memory cell cannot be diminished in proportion to an increase in integration of semiconductor elements. As a result, as long as a silicon oxide film is employed as a dielectric material, it is inevitable that the area occupied by a capacitor in a memory cell becomes increasingly larger as the integration is further advanced.
On the other hand, there has been extensively studied to employ a thin film having a high dielectric constant as a dielectric film of capacitor in a memory cell of DRAM. The purpose of using the thin film having the high dielectric constant as a dielectric film of capacitor is to minimize the area of capacitor so as to miniaturize a high integrated memory cell. If a thin film having the high dielectric constant is to be employed as a dielectric film of capacitor in a memory cell of DRAM, the quantity of electric charge that can be stored per unit area is required to be large and at the same time a leakage current is required to be small. Furthermore, for the purpose of operating a memory at a high speed, it is desired that both charging and discharging can be performed at a sufficiently high speed.
Under the circumstances, the employment of Ta.sub.2 O.sub.5 (relative dielectric constant: about 28) has been studied for the application thereof to a memory cell of DRAM in place of the conventional dielectric films, i.e. silicon oxide (SiO.sub.2) film (relative dielectric constant: about 4) and silicon nitride (Si.sub.3 N.sub.4) film (relative dielectric constant: about 7). However, the magnitude of the dielectric constant of Ta.sub.2 O.sub.5 is still insufficient to meet the demand for increasing the integration by minimizing the area of capacitor.
Accordingly, the employment of strontium titanate (SrTiO.sub.3) or of barium strontium titanate (Ba.sub.x Sr.sub.1-x TiO.sub.3) has been extensively studied as a candidate for a dielectric material having a higher dielectric constant. The (Ba, Sr) TiO.sub.3 is formed of a solid solution comprising BaTiO.sub.3 and SrTiO.sub.3, forming a perovskite crystal structure. It is known that SrTiO.sub.3 is a paraelectric material, and BaTiO.sub.3 is a ferroelectric substance having a Curie temperature of about 120.degree. C.
Since Sr and Ba are less evaporable as compared with Pb or Bi, the control in composition of (Ba, Sr) TiO.sub.3 is relatively easy in the formation of a thin film of (Ba, Sr) TiO.sub.3. Moreover, when these BaTiO.sub.3 and SrTiO.sub.3 are crystallized, there is a little possibility of these crystals take any other crystal structure (such as pyrochlore structure) than the perovskite crystal structure.
There is a problem however in (Ba.sub.x Sr.sub.1-x TiO.sub.3) that when this compound is formed into a thin film of not more than 100 nm in thickness, the dielectric constant thereof is lowered, thus making it impossible to store a sufficient electric charge therein.
Meanwhile, a memory device employing a ferroelectric substance as a capacitor (ferroelectric memory) is now being developed as a memory medium. As a matter fact, some ferroelectric memories have been actually put into practical use. Since the ferroelectric memory is non-volatile, the memory once stored in the memory would not be dissipated even after a power source is turned off. Moreover, the ferroelectric memory is featured in that the inversion of spontaneous polarization is very fast if the film thickness thereof is sufficiently thin, so that a high speed writing and reading which is comparable to that of DRAM is possible. Furthermore, since the ferroelectric memory cell of one bit can be constructed with a single transistor and a single ferroelectric capacitor, it is suited for manufacturing a large storage device.
It is also studied to operate a ferroelectric memory in a manner of DRAM. Namely, in this case, the ferroelectric substance is employed as a memory cell capacitor of the DRAM without allowing the spontaneous polarization of the ferroelectric substance to be inverted in the ordinary operation. However, the ferroelectric memory is used, by taking advantage of a residual polarization of the ferroelectric thin film, as a non-volatile memory only at a moment prior to the turn-off of the power of apparatus. This method is effective in minimizing the effect of a phenomenon (fatigue) that the ferroelectricity of the ferroelectric memory is gradually deteriorated with a repetition of the polarization inversion, which has been considered to be most the serious problem of the ferroelectric memory.
It is required for a ferroelectric thin film suited for use in a ferroelectric memory to have various features, such as being large in residual polarization, low in temperature dependence of residual polarization, minimal in deterioration (fatigue) of residual polarization resulting from the repetition of polarization inversion, and capable of maintaining the residual polarization for a long period of time (retention). Additionally, for the purpose of actuating the memory at a high speed, the inversion of spontaneous polarization is required to be sufficiently rapid.
At present, lead zirconate titanate (PZT) has been mainly employed as a ferroelectric material. This PZT is formed of a solid solution comprising lead zirconate (PbZrO.sub.3) and lead titanate (PbTiO.sub.3). In particular, a PZT which is formed of a solid solution comprising lead zirconate and lead titanate at a ratio of approximately 1:1 in molar ratio is considered to be large in spontaneous polarization and capable of inverting even at a low electric field, and hence considered to be excellent as a memory medium. Since the transition temperature (Curie point) between ferroelectric phase and paraelectric phase is relatively high, i.e. about 300.degree. C., there is little probability that a data stored in the memory is vanished by heat as long as the memory is kept within a range of temperature (not more than 120.degree. C.) which the ordinary electronic circuit would encounter.
However, the manufacture of a thin film of the PZT which is excellent in quality is known to be difficult. First of all, since lead (Pb) constituting a main component of the PZT easily evaporates at a temperature of 500.degree. C. or higher, an accurate control of the composition of PZT is very difficult. Secondly, although it is known that the PZT exhibits a ferroelectricity only when the PZT takes a perovskite crystal structure, it is very difficult to obtain a PZT film having a perovskite crystal structure. Instead, it is easy to obtain a PZT having a pyrochlore structure, which however does not exhibit ferroelectricity. In order to obtain a PZT film of perovskite crystal structure, a treatment at more or less high temperature (about 500.degree. C.) is required. However, if the PZT film is treated at such a high temperature, Pb is caused to evaporate or diffuse as mentioned above, thereby making it difficult obtain a PZT film of desired composition.
Recently, research on SrBi.sub.2 Ta.sub.2 O.sub.9, which is one of Bi series layer perovskite compounds, is being extensively performed in an effort to apply it to a ferroelectric memory, etc. However, although Bi is an element having a low melting point just like Pb, it is required to be sufficiently crystallized for attaining a hysteresis, i.e. it is required to be heat-treated at a high temperature (700.degree. C.). The heating of the compound at such a high temperature cannot be carried out without inviting various problems that Bi would be evaporated or diffused into electrodes or other portions of device. Additionally, in spite of the fact that this compound is highly anisotropic in crystallinity, when this compound is desired to be used in the form of a non-orientated polycrystalline film, the problem of non-uniformity in ferroelectricity among the finely processed films would be raised.
In spite of such a difficulty as mentioned above in the manufacture of a PZT thin film or Bi series layer compound thin film of high quality with good reproducibility, these thin films are still extensively studied as a candidate for a memory medium (capacitor), because there is no other ferroelectric material other than PZT and Bi-based compounds which is suited for use as a memory capacitor.
Except for PZT, barium titanate (BaTiO.sub.3) is known to exhibit ferroelectricity at room temperature. This BaTiO.sub.3 is known as having the same perovskite crystal structure as that of PZT and a Curie point of about 120.degree. C. Since Ba is less evaporable as compared with Pb, the control of composition in the manufacture of a thin film of BaTiO.sub.3 is relatively easy. Moreover, once this BaTiO.sub.3 is crystallized, there is little possibility that this BaTiO.sub.3 takes any other crystal structure (a pyrochlore structure) than the perovskite crystal structure.
In spite of these advantages, the BaTiO.sub.3 for use as a thin film capacitor has not been seriously taken up for study as a candidate for a memory medium of ferroelectric memory. One of the reasons for this can be ascribed to the facts that the residual polarization thereof is small as compared with PZT and the temperature dependence of the residual polarization is relatively large. In other words, the reason may be ascribed to the facts that the Curie temperature Tc of the BaTiO.sub.3 is inherently relatively low (about 120.degree. C.), and the coercive electric field of the BaTiO.sub.3 is relatively low. In this case, the Curie temperature Tc means a temperature at the moment of phase transition from the ferroelectric phase to the paraelectric phase, which is inherent to a ferroelectric material so that a material, even exhibiting ferroelectricity at room temperature, would not exhibit ferroelectricity at a temperature higher than the Curie temperature. Therefore, when a ferroelectric memory is manufactured by making use of a thin film capacitor consisting of BaTiO.sub.3, not only there is a probability that the data stored therein may be vanished when the memory is once exposed to a high temperature (120.degree. C.) for some reason, but also the temperature dependence of residual polarization is relatively large even at a temperature range (85.degree. C. or less) to which an electronic circuit is generally exposed, thus making the operation of the memory unstable.
Therefore, it has been considered that a thin film capacitor formed of a ferroelectric thin film consisting of BaTiO.sub.3 is not suited for use as a memory medium of a ferroelectric memory.
Meanwhile, it has been recently reported a phenomenon that when a BaTiO.sub.3 film was epitaxially grown to a thickness of 60 nm on a Pt/MgO single crystal substrate, the Curie temperature thereof was raised up to 200.degree. C. or more (K. Iijima, Appl. Phys., Vol.62, No.12 (1993), pp.1250-1251). This document reports that this phenomenon is assumed to have been brought about by a shrinkage of a-axis and an extension of c-axis of BaTiO.sub.3 perovskite lattice, since the BaTiO.sub.3 film has been epitaxially grown in conformity with the lattice constant of Pt. This document further goes on saying that this phenomenon can be observed only when the thickness of the BaTiO.sub.3 film is not more than 60 nm, and that if the thickness of the BaTiO.sub.3 film exceeds over 60 nm, the lattice constant of the BaTiO.sub.3 film would be relaxed to that inherent to the BaTiO.sub.3 due to a misfit dislocation (due to an increase of film thickness to larger than the critical film thickness).
On the other hand, it is known that if a thickness of a ferroelectric thin film is not more than 1 .mu.m, the residual polarization becomes smaller with a decrease in film thickness of the ferroelectric thin film. As a matter of fact, the aforementioned document reported that when the film thickness of the BaTiO.sub.3 epitaxial film was 100 nm or less, the residual polarization thereof was limited to not more than 2 to 3 .mu.C/cm.sup.2. Therefore, even if it is possible with the employment of a BaTiO.sub.3 epitaxial film having a thickness of 60 nm or less to raise the Curie temperature, it is impossible to attain a sufficient residual polarization for practical use as a ferroelectric thin film.
Because of these reasons, it has been considered that even if it is possible with the employment of the conventional BaTiO.sub.3 thin film capacitor to raise the Curie temperature by way of epitaxial effect, it is very difficult to actually use it as a memory medium for use in a ferroelectric memory.
With a view to solve the aforementioned problems involved in the conventional ferroelectric thin film, the present inventors have discovered that, if a dielectric material (for example, Ba.sub.x Sr.sub.1-x TiO.sub.3) having a lattice constant which is relatively close to that of a lower electrode (for example, Pt) is employed and at the same time if an RF magnetron sputtering method which is comparatively effective in avoiding misfit dislocation during the deposition of film is adopted as a method of forming a film, it is possible, even if the thickness of the film is as thick as about 200 nm, to maintain a state of film where the lattice constant is extended in the direction of film thickness (c-axis) while the lattice constant in-plane of film (a-axis) is shrunk as compared with those of the inherent lattice constant which can be obtained through an epitaxial effect. As a result, it has been confirmed that it is possible to shift the ferroelectric Curie temperature Tc toward a higher temperature side and hence to obtain a ferroelectric thin film which is capable of exhibiting a large residual polarization at room temperature and maintaining a sufficiently large residual polarization even if the temperature of the film is raised up to about 85.degree. C.
For example, it has been experimentally confirmed by the employment of Pt (lattice constant a=0.39231 nm) which is hardly oxidized as a lower electrode and strontium barium titanate (Ba.sub.x Sr.sub.1-x TiO.sub.3 : as BST; x=0.44 to 0.90) as a dielectric substance that this dielectric substance is capable of exhibiting ferroelectricity even if the composition thereof is shifted to a compositional region (x.ltoreq.0.7) where ferroelectricity is not expected to be exhibited inherently at room temperature, and that when this dielectric substance is in the compositional region where ferroelectricity can be inherently exhibited at room temperature, the Curie temperature thereof can be further raised beyond the inherent Curie point (which is higher than room temperature), indicating that preferable ferroelectric properties for practical use can be realized.
The present inventors have further found out that a method of introducing a strain to a ferroelectric substance by taking advantage of mismatching in lattice constant between a lower electrode and the ferroelectric substance is also effective for improving a dielectric constant of paraelectric thin film. Namely, it has been found out that when a thin film of Ba.sub.x Sr.sub.1-x TiO.sub.3 (where x (the content of Ba)=about 0.24) is epitaxially grown on a Pt layer, a phenomenon of a prominent increase in dielectric constant can be observed even though no ferroelectricity can be observed. Therefore, it is possible by taking advantage of this phenomenon to greatly increase the accumulation of electric charge in a dielectric film to be employed for a memory cell of DRAM.
However, it has been found by the latest experiments made by the present inventors that when an epitaxial film of Ba.sub.x Sr.sub.1-x TiO.sub.3 as a dielectric substance is deposited on a lower electrode so as to obtain the aforementioned epitaxial effects by taking advantage of mismatching of lattice constant, i.e. to provide a paraelectric material with ferroelectricity; to provide a ferroelectric material with an enhanced ferroelectricity; or to provide a paraelectric material with an increased dielectric constant, various problems would be raised in the operation of device at a high speed.
Namely, it has been found that when a film is formed of a ferroelectric substance, the D-E hysteresis depicted with a higher frequency may indicate a lower residual polarization as compared with that where the D-E hysteresis is depicted with a lower frequency. Further, when a film is formed of a paraelectric substance, the frequency dependence of a dielectric constant becomes larger, so that a phenomenon that dielectric constant is lowered with an increase in frequency may be seen.